I am not going to argue theologically about the Filioque (Latin expression, meaning 'and the Son' in plain English, that regards procession of The Holy Ghost). I am going to prove from a document of an early ecumenical council of The Church that the Orthodox there taught 'Filioque' and anathematized anti-Filioque doctrine.

"If anyone defends the heretical writings of Theodoret which were composed against the true faith, against the first holy synod of Ephesus and against holy Cyril and his Twelve Chapters ... let him be anathema." (2nd ecumenical council of Constantinople, 13th anathema of "the Three Chapters").

What did the holy Cyril (of Alexandria) write in his Twelve Chapters (Anathemas) regarding Filioque and what heretical writing did the Theodoret compose against the holy Cyril (of Alexandria) regarding Filioque?

Holy Cyril of Alexandria wrote this:
"9. If any man shall say that the one Lord Jesus Christ was glorified by the Holy Ghost, so that he used through him a power not his own and from him received power against unclean spirits and power to work miracles before men and shall not rather confess that it was his own Spirit through which he worked these divine signs; let him be anathema." (here).

And Theodoret replied this:
"Against IX. ... We shall confess that the Spirit of the Son was His own if he spoke of it as of the same nature and proceeding from the Father, and shall accept the expression as consistent with true piety. But if he speaks of the Spirit as being of the Son, or as having its origin through the Son we shall reject this statement as blasphemous and impious. For we believe the Lord when He says, "The spirit which proceedeth from the Father;"" (here).

If having read these words you didn't fall to the ground then you know next to nothing about the Filioque issue. The bold words of Theodoret are to the letter the words of Photius and the Easterns** who followed him (thereby also called Photians) against the Filioque.

** - due to the above error and other errors against the orthodox faith the Photians have lost Orthodox name and by The Church have been referred to only as the Greeks (as regards Churches, "Greek" is the synonym of the East as "Latin" ("Roman") is the synonym of the West: "in ritual and canon law the Latin or Roman school formed the West. In a still broader sense the East may be called Greek. True, many Eastern Churches know nothing of Greek; the oldest (Nestorians, Armenians, Abyssinians) have never used Greek liturgically nor for their literature; nevertheless they too depend in some sense on a Greek tradition. ..." (here) ) or Easterns in general and Greeks (Byzantines), Copts, Armenians etc. (according to their rite) in particular.

And now you see that the words of Theodoret, Photius and the Photians against the Filioque have been anathematized by the Orthodox Fathers of the 2nd ecumenical council of Constantinople in 553 and that the holy Cyril of Alexandria himself taught the Filioque!

No wonder then that a fiery defender of the orthodox faith, holy Athanasius, contemporary of holy Ambrose, didn't say a word against him when he openly wrote the Filioque (here, 119 and 120), that St. Cyril of Alexandria wrote no anathema against his contemporary, holy Augustine, when he also openly wrote the Filioque (here, chapter 14), and that none of the first seven ecumenical councils of The Church anathematized the Filioque or the Filioque of holy Ambrose or holy Augustine.

No follower of Photius would dare to say that the 2nd ecumenical council of Constantinople wasn't orthodox and differed from the ecumenical councils of Nicea, Constantinople, Ephesus and Chalcedon. Necessarily follows that all these councils also teach the Filoque and anathematize the words of Theodoret, Photius and Photians against the Filioque and that the Orthodox Fathers have nothing to do with the anathematized Photians who abuse their Orthodox name.

The next objection against the Filioque is its allegedly illegal addition to the Nicene-Constantinople Creed.

Here under note 72 it is written:
"Conc. Ephes. Can. VII. 'The holy Synod has determined that no person shall be allowed to bring forward, or to write, or to compose any other Creed (ἑτέραν πίστιν μηδενὶ ἐξεῖναι προφέρειν ἤγουν συγγράφειν ἢ συντιθέναι), besides that which was settled by the holy fathers who assembled in the city of Nicæa, with the Holy Spirit. But those who shall dare to compose any other Creed, or to exhibit or produce any such, if they are bishops or clergymen, they shall be deposed, but if they are of the laity, they shall be anathematized.' ..."

Greek "πίστιν" transliterated as "pistin" means "faith" (here), NOT "creed" (see and listen here how "Creed" is written and pronounced in Greek) as falsely stated by the above note 72 and by all enemies of the Filioque and/or of addition of the Filioque to the Nicene-Constantinople Creed.

So, it is a huge lie that composition of a new Creed or change of an existing Creed was forbidden. Orthodox composition of a new creed or orthodox change of an existing Creed has always been allowed. (1)

Now, it was proven above that the Filioque belongs to the orthodox faith and not to another faith. (2)

From (1) and (2) necessarily follows that addition of the Filioque to the Nicene-Constantinople wasn't heterodox nor illegal but was legal and absolutely necessary in order to protect The Church from the anti-Filioque heresy of Theodoret and Photius (see above).